Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 class CheckedModeHelper {
   final String name;
 
   const CheckedModeHelper(String this.name);
@@ skipping 102 matching lines @@
     DartType type = node.typeExpression;
     Element element = type.element;
     String isField = codegen.backend.namer.operatorIs(element);
     arguments.add(js.string(isField));
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
     String asField = codegen.backend.namer.substitutionName(element);
     arguments.add(js.string(asField));
   }
 }